This invention relates generally to a tuning device for a stringed musical instrument and to the method of assembling the tuning device.
There are many known devices of the worm and pinion type for varying the tension in the string of a musical instrument. Some of these devices are disclosed in U.S. Pat. Nos. 3,564,573 and 2,356,766. It is desirable to provide pinion shafts which are free of wobble and are constrained to pure rotary motion. This is because a string tuned with a wobbly pinion shaft does not produce a clear tone. The vibration of the string causes the shaft to move in the housing if it is not firmly supported; this causes the length of the string to vary slightly and thus produces a tone that is not clear. The distortion is especially noticeable when the instrument is played through an amplifier.
Any tuning device of the worm and pinion gear type must provide means for holding the pinion gear against axial motion. In some known devices the pinion gear is integrally formed with the shaft around which the string is wound. In these devices, the pinion and shaft are inserted in the housing until one side of the pinion gear abuts an annular bearing surface in the housing. The opposite surface of the pinion gear abuts a cap which is pressed into the housing after the pinion gear is in place. Thus the pinion gear is trapped against axial motion by two bearing surfaces. This method of holding the pinion gear against axial motion can fail when the cap becomes loosened in the housing.
In another approach to securing the pinion gear against axial motion, the pinion gear and the string shaft are separate pieces, held together by a machine screw. In this type of device a shoulder on the shaft abuts a bearing surface on the housing to prevent axial motion in one direction. The pinion gear abuts an annular bearing surface in the housing to prevent axial motion in the other direction. A disadvantage to this approach is eccentricity of assembly, accumulated manufacturing tolerances and that the pinion gear and shaft separate when the machine screw comes loose.
Additionally the desirability of eliminating backlash between the worm and pinion gear of tuning devices has been recognized. An extremely tight fit between the worm and pinion causes the gear to jam, and a loose fit causes excess play; both conditions are detrimental to sensitivity of tuning. There are known solutions to the problem of proper gear adjustment, one of which is disclosed in U.S. Pat. No. 4,014,239.